This invention relates to a cryogenic process for separating synthesis gas by partial condensation.
Synthesis gas is commonly produced in the chemical processing industry by a variety of techniques, for example, the steam reforming of natural gas (methane), and the pyrolysis or partial oxidation of both solid and liquid hydrocarbon feedstocks. The so-produced synthesis gas mixture contains the desired hydrogen and carbon monoxide products, as well as residual methane. By way of example, the synthesis gas mixture may contain approximately 40 to 70 mole % hydrogen, 15 to 60 mole % carbon monoxide, 0.1 to 15 mole % methane and the remainder argon and nitrogen. Consistent with the use of the synthesis gas constituents as chemical precursors, the three major components are separated and purified to the required degree. Both cryogenic and noncryogenics processes are available to provide the required separations and each offer their own advantages. The present invention is concerned primarily with the cryogenic approach.
In the past, there have been two basic cryogenic approaches to the complete separation of synthesis gas; the methane wash approach and the partial condensation - pressure swing adsorption approach. In the methane wash approach, a synthesis gas feed stream is provided at elevated pressure and cooled to form a vapor-liquid mixture which is introduced to the methane wash column. Prior to the methane wash column, the feed gas stream may undergo a preliminary separation by partial condensation to increase its hydrogen content. In the wash column, the feed is contacted with a high purity, sub-cooled methane wash liquid for absorption of feed carbon monoxide into the methane wash liquid. High purity hydrogen product is recovered from the overhead of the methane wash column and liquid containing the wash methane and absorbed carbon monoxide is recovered in the bottoms. Recovered bottoms liquid is then throttled to reduced pressure and fractionated in a carbon monoxide separation column. This column produces an overhead carbon monoxide product and a high purity methane bottoms liquid. A portion of the methane bottoms is then subcooled and recycled as wash liquid for the methane wash column.
Current examples of the methane wash approach are those described in Allam et al. U.S. Pat. No. 3,86,756 and Martin U.S. Pat. No. 4,102,659. In such systems, the final hydrogen purification is done within the cryogenic equipment by absorbing heavy components from the light stream with a subcooled methane wash stream. The high purity methane wash stream is generated in the carbon monoxide separation column where the binary separation of carbon monoxide from methane is conducted. Because of the necessity for generating a sizable, sub-cooled liquid methane wash stream, these processes tend to be very energy intensive. Moreover, these systems are also limited to synthesis gas feeds having low carbon monoxide to methane molar ratios, e.g., less than about 30 and preferably lower. At higher carbon monoxide to methane molar ratios, the power requirements associated with refrigeration and the complexity associated with this approach cannot normally be justified relative to the partial condensation - PSA approach.
A typical commercially practiced prior art system employing the partial condensation approach to synthesis gas processing represented by Pryor U.S. Pat. No. 3,508,413 (FIG. 1). In the Pryor process, the synthesis gas is first cooled and partially condensed in a first heat exchanger and the condensed liquid fraction is separated from the vapor fraction. This first separation occurs at an intermediate cryogenic temperature and reduces the amount of hydrogen which is subsequently dissolved in the liquid phase produced upon complete cooling. Earlier less efficient prior art approaches cooled the feed gas stream directly to the temperature at the cold end of the system. This dissolves a large amount of hydrogen in the liquid phase subsequently recovered, and results in substantial difficulty in separating this dissolved hydrogen from the final carbon monoxide product.
The non-condensed feed gas recovered from the first separator is then further cooled in a second lower temperature heat exchanger and a hydrogen enriched gas is separated from a carbon monoxide rich liquid in a coldest temperature separator. The hydrogen rich gas from this separation is then partially warmed in the second heat exchanger, expanded through a turbine to develop refrigeration for this system and then its sensible refrigeration is consecutively removed through the second and first heat exchangers respectively. The liquid recovered from the first separator with liquid from an intermediate temperature separation are thereafter throttled into a lower pressure separator. Similarly, the hydrogen saturated liquid recovered from the lowest temperature separator is partially reboiled in the lowest temperature heat exchanger and is throttled into still another separator. The hydrogen saturated liquid fractions subsequently recovered from both separators are expanded into a carbon monoxide-methane separation column. The overhead saturated vapor streams recovered from each of these separators are combined, warmed in the first heat exchanger and then recycled to join the synthesis feed gas. It should be noted that one portion of the saturated liquid from the last-mentioned separator is flashed into a subatmospheric pressure separator. This operation provides an additional source of refrigeration for the system. The patentee also claims that this step greatly increases the thermal efficiency of the cycle and therefore greatly reduces the power requirements of the process by reducing the temperature difference at both the warm and cold ends of the first heat exchanger. The liquid recovered from the sub-atmospheric pressure separator is fed to the carbon monoxide-methane separation column while the vapor separated is passed to the carbon monoxide product stream. Reboil for the carbon monoxide-methane separation column is provided by heat exchange with the cooling synthesis feed gas stream. The column produces a methane-enriched fuel gas bottoms product and the high purity carbon monoxide overhead product.
A prior art modification of the Pryor partial condensation system is illustrated in FIG. 1. The synthesis gas supplied at super atmospheric pressure is supplied through conduit 11 and first cooled and partially condensed in first heat exchanger 12, and the condensed liquid fraction is separated from the vapor fraction in first separator 13. The non-condensed vapor fraction is passed through conduit 14 for further cooling in second lower temperature heat exchanger 15 and a hydrogen enriched gas is separated from a carbon monoxide rich liquid in lowest temperature separator 16. The hydrogen rich gas in conduit 17 is then partially rewarmed in heat exchanger 15, expanded through turbine 18 to develope refrigeration for the system and passed back through heat exchangers 15 and 12 for recovery of sensible refrigeration.
The hydrogen-saturated first liquid fraction is discharged from first separator 13 into conduit 19 and throttled through valve 20 into first lower pressure separator 21. The throttled low pressure carbon monoxide enriched gas is discharged through conduit 22 for sensible refrigeration recovery in first heat exchanger 12. The throttled carbon monoxide and methane enriched liquid is passed through conduit 23 to an intermediate level of carbon monoxide-methane separation column 24.
The carbon monoxide-enriched liquid fraction from coldest separator 16 is discharged through circuit 25, throttled in valve 26 and partially rewarmed (reboiled) in lower temperature heat exchanger 15 before entering second lower pressure separator 27. The carbon monoxide enriched gas from the latter is discharged therefrom in conduit 28 and joins gas from conduit 22 in combination conduit 29 for recovery of sensible refrigeration in first heat exchanger 12. The carbon monoxide rich liquid fraction from second lower pressure separator 27 is passed through bottom conduit 30 into the top end of carbon monoxide-methane separation column 24 as reflux liquid.
Heat for driving the lower end of column 24 is provided by a warmer fluid in reboiler 31, as for example, a partially cooled portion of the synthesis feed gas (not illustrated). The methane rich bottoms liquid from column 24 is withdrawn through conduit 33 and passed through first heat exchanger 12 for recovery of latent refrigeration. The high purity carbon monoxide overhead product gas from column 24 is discharged through conduit 32 and passed to first heat exchanger 12 for recovery of its sensible refrigeration.
The main difference between the Pryor and FIG. 1 prior art partial condensation systems is that the latter eliminates the intermediate temperature separator and the sub-atmospheric pressure separator along with the associated valves and piping.
There are two principle disadvantages of both of these prior art partial condensation systems. Firstly, the carbon monoxide rich liquid fraction from the lowest temperature separator 16 passes through second lowest pressure separator 27 and is coupled directly to separation column 24, i.e. the operating pressure of separator 27 dictates the maximum operating pressure of the column. Since a low pressure warming stream yields a desirable reboiling characteristic in lower temperature heat exchanger 15, with the use of a low recycle flow, the bottoms from separator 16 must be flashed to a low pressure. Accordingly, the carbon monoxide-methane separation column must also operate at a low pressure. Attempts at processing stream 25 at higher pressures result in a higher recycle flow and subsequently higher recycle compression requirements, as well as a performance degradation in lower temperature heat exchanger 15.
The second deficiency of these prior art partial condensation systems relates to throttling of the hydrogen saturated liquid from first separator 13 into lower pressure separator 21. This throttling step is conducted at a low pressure to insure complete removal of hydrogen. However, this also results in a high carbon monoxide loss into the overhead vapor, thereby greatly increasing the power requirements due to recycle compression.
A further limitation of these prior art partial condensation processes is that they cannot be adapted to separation of synthesis gas mixtures having carbon monoxide to methane molar ratios below about 10. This is because they cannot optimally balance the competing requirements of refrigeration, product recovery and purity.
An object of this invention is to provide an improved cryogenic process for separating synthesis gas by partial condensation, in which the operation of the carbon monoxide-methane separation is uncoupled from the low pressure reboiling carbon monoxide stream in the lower temperature heat exchanger.
Another object is to provide such a process which permits operation of the carbon monoxide-methane separation column at the optimum pressure without sacrificing overall process efficiency.
Still another object is to provide an improved cryogenic process for separating synthesis gas by partial condensation, which process requires substantially less total power than presently available partial condensation processes.
An additional object is to provide such a process which extends the advantages of the partial condensation approach for synthesis gas separation to feed gas streams having carbon monoxide to methane molar ratios below about 10.
Other objects will be apparent from the ensuing disclosure and appended claims.